Glass is typically processed by batch heating and refining within a melting furnace. Glass batches are typically heated via burners, which serve as the primary heat source, and from glass melt electrodes embedded in the wall of the melting furnace. The number of electrodes depends upon the size of the melting furnace and the characteristics of the glass being processed. These glass melt electrodes introduce additional thermal energy into the furnace by passing a current through the glass melt. Due to the extremely high temperatures required for glass melting (typically between approximately 1100° C. and approximately 1700° C.), glass melt electrodes are typically formed of refractory metals, such as molybdenum (Mo) or tungsten (W), or alloys thereof. However, molten glass is often extremely corrosive and oxidative, and even such refractory metals may corrode and degrade after long periods of use.
Conventional electrodes are polycrystalline and typically have grain sizes no greater than 100 microns. One primary corrosion mode in conventional electrodes is corrosive attack on the refractory metal along the crystalline grain boundaries thereof. Due to the increased interatomic disorder and free volume at grain boundaries, corrosive elements (particularly polyvalent elements such as Pb, As, Sb, Co, Ni, or Mn) from the glass melt diffuse more quickly into the melt electrode at the grain boundaries, leading to increased corrosion. This effect may be mitigated somewhat by the incorporation of alloying elements (e.g., Si, B, Pd, Pt, Ir, or Ru) within the electrode material, as the alloying elements tend to preferentially accumulate at the grain boundaries and retard grain-boundary diffusion and related corrosive attack. However, such alloying elements may be quite expensive and may also diminish the mechanical processability of the electrode material, making electrode formation more time-consuming and expensive. In some cases, the alloying elements may deleteriously contaminate the molten glass itself. Such considerations limit the amount of alloying elements that may be added to the electrode material, and thus grain boundary diffusion (and concomitant corrosion) typically still occurs in conventional electrodes.
In view of the foregoing, there is a need for improved glass melt electrodes that resist corrosion but remain amenable to mechanical processing.